1. Field of the Invention
This invention relates to a method for synchronizing the devices on the receiving side with those on the transmitting side of a transmission system operating on a time multiplex or code multiplex basis. In particular, such a system utilizes the SSMA technique where the information is transmitted in a broad frequency range, compared with the base band which is occupied by the information. Furthermore, this invention relates to improved apparatus for the transmission and receiving sides of a transmission system for the execution of the method.
2. Description of the Prior Art
In transmission techniques, one distinguishes, among other things, transmission systems which operate on a time multiplex basis from transmission systems which operate on a code multiplex basis when connections are to be established between several subscribers. It is characteristic for time multiplex systems that the individual transmission bursts are transmitted with respect to time so that they do not interleave at the summing point of all signals while with the code multiplex technique, the individual transmission channels are steadily in operation and can be distinguished from one another by different code signals. A typical time multiplex method is the so-called "TDMA" technique as it is provided for the application with satellite transmission. A typical code method is the so-called SSMA technique, and this invention is of particular importance for the SSMA technique. The expression "SSMA" is derived from the term "Spread-Spectrum-Multiple-Access Modulation". The SSMA method is primarily applied with satellite transmission paths having multiple access. This technique has, for example, been described in detail in the publication "Proceedings of the IEEE", Volume 54, 1966, pages 763-777. It is essential with the SSMA technique that a fairly large number of transmission stations operate in the same radio frequency range and that signs which are transmitted by the individual stations respectively have a particular code modulation unique unto themselves. This code modulation has two objectives, namely, to expand the individual signal to a larger frequency range and to render the signal recognizable as a code sign for a certain receiving station. Thus, it becomes possible, with respect to the individual receiver, to screen out a certain transmitting station from the frequency spectrum which is received on the basis of its code sign. The code sign of the individual stations is therefor a relatively long sign which includes, for example, ten mega bits. Most often, the individual sign is expressed by means of a phase change of the transmitted high frequency oscillation with respect to the respectively preceding bit. The information itself is additionally impressed onto the code sign in the transmitting station, in particular by the application of a phase shift modulation in such a way that the code sign, which includes relatively many bits, is inverted in its phase with respect to essentially fewer bits when a sign change occurs in the information flow. On the receiving side, the code signal is produced in a code generator which runs synchronously with the transmitting side, and submitted to a multiplication process with the received signal in the radio frequency position or, which is most often the case, in the intermediate frequency position. Due to the multiplication process, the signal power will only appear in the form of an individual spectrum line when the code is received which has not been changed by any additional information. If the code signal is changed on the transmitting side by means of inverting individual phase shift changes in the rhythm of the information flow which, compared with the code signal, only comprises a few bits, information will be obtained from the individual frequency component when the unchanged code signal is received which comprises all frequency components corresponding to the information on the transmitting side. If several transmitting stations operate jointly and simultaneously in the same frequency range, additional frequencies will occur after the multiplication process; however, the power related thereto is usually less than that of the useful signal and such signals can be grouped as noise with respect to the desired information. Therefore, devices for producing a phase-modulated electric oscillation are required with the SSMA technique, both on the transmitting side and the receiving side. However, an essential problem with such devices is that the code signal which is produced in the receiver must be phase synchronous with the code signal which is recorded in the receiver and which comes from the transmission station. The fact that a correct phase position of the output power of a multiplier increases strongly in a narrow frequency range is utilized as recognition criterion for the correct phase position with SSMA receivers. Therefore, a threshold circuit is provided with these receivers in the output of the multiplier from which the criterion for the correct synchronization is derived when a certain threshold value of the output signal is exceeded.